Photomultiplier tubes are commonly employed for detecting radiation and are found in a diverse range of applications including those related to spectroscopy, particle physics, astronomy, medical imaging and diagnostics, and laser ranging. Photomultiplier tubes are remarkable for their sensitivity, and in some situations can detect a single photon impinging on the photosensitive area of the photomultiplier tube. In addition, photomultiplier tubes are often favored over other types of detectors due to their high responsivity and low-noise characteristics. Further, photomultiplier tubes can be made with relatively large photosensitive areas which is advantageous in certain applications.
The operation of a photomultiplier tube is explained with reference to FIG. 1. FIG. 1 shows a typical photomultiplier tube 102 comprised of various electrodes situated inside an evacuated enclosure as delimited by a metal tube 104 that is sealed at one end with a stemplate 106 and at the opposite end with a transparent faceplate 108 or a glass envelope. There are many variations on the structure of photomultiplier tubes and on specific function of the components of photomultiplier tubes. The present invention is not specific to a particular type of photomultiplier tube, and is of general applicability to all photomultiplier tubes including those with focusing electrodes, multiple anodes, and multiple photocathodes. Therefore, the following description will be limited to aspects of photomultiplier tube structure and operation that are relevant to the invention and that are common to many types of photomultiplier tubes.
In FIG. 1, incident radiation 110 is admitted through a glass faceplate 108 or glass envelope to impinge on a radiation-sensitive photocathode 112. This causes the emission of one or a few electrons from the photocathode. A voltage bias provided by a voltage source 114 connected between the photocathode 112 and a dynode 116 in close proximity to the photocathode 112 creates an electric field between the photocathode 112 and said dynode 116. The electric field accelerates the electrons 118 that are emitted by the photocathode 112 in response to the radiation 110 incident upon it. The strength and polarity of applied voltage source 114 connected between the photocathode 112 and dynode 116 is such that the direction of the electric field forces electrons emitted from the photocathode to impact 116 with sufficient energy to cause the emission of additional secondary electrons 122. The electrons emitted from dynode 116 are accelerated toward a second nearby dynode 124 by an electric field created by a voltage imposed by voltage source 120 between dynode 116 and the second dynode 122. Upon impact with the second dynode 124, a number of electrons 128—greater in number than the secondary electrons 122 emitted from dynode 116 that impact 124—are emitted from dynode 124 and impact dynode 130. This process is repeated according to the number of dynodes, up to and including 132, creating a cascade of electrons that increases in number between successive dynodes. The electron cascade from the ultimate dynode 132 terminates when the electrons 134 emitted from 132, which is biased with respect to ground potential by voltage source 136, impact an anode 138, thereby inducing a current 140 in the anode that is sensed by an ammeter 142 or other external circuit. The anode current 140 thus serves as an indicator and measure of the radiation incident upon the photocathode.
It is noted that in order to create and sustain the electron cascade that is generated in response to absorption of radiation in the photocathode, each electrode (photocathode 112, dynodes 116, 124, 130, 132 and similar dynodes not shown, and anode 138) has a distinct and appropriate voltage bias with respect to adjacent electrodes.
It is further noted that the electron cascade constitutes an electric current between adjacent electrodes, that the electron cascade (118, 122, 128, 134 and corresponding electron cascades between electrodes not shown in FIG. 1) is amplified in that said electron current associated with each dynode pair increases proceeding from the photocathode to the anode. Moreover, the current between electrode pairs and the current supplied to each electrode varies with the intensity of radiation absorbed by the photocathode.
The use of separate independent voltage supplies, i.e., 114, 120, 126, 130 and the like, to bias the various electrodes as indicated in FIG. 1 is not commonly done due to the expense and complexity of using several high-voltage sources. Photomultiplier tubes often have between ten and twenty dynodes, and such a method of biasing would thus necessitate ten to twenty independent high-voltage sources. More typically, a single high-voltage source is utilized in combination with a voltage divider network that creates a succession of voltage levels. Each voltage level created by the voltage divider network is used to bias an electrode of the photomultiplier tube.
A simple recourse for realizing a voltage divider circuit is a passive network of resistors and capacitors. It is instructive to discuss the issues and performance limitations of such a resistance network in order to appreciate the advantageous features of the present invention. FIG. 2 shows a resistor divider network that produces a ladder of voltage levels that can be tapped to bias the photomultiplier tube electrodes as shown. The photomultiplier tube 202 of FIG. 2 is otherwise identical to that shown in FIG. 1, and includes a metal tube 204, a stemplate 206, a glass faceplate 208, a photocathode 210, a series of dynodes (212, 214, 216, 218, 220), and an anode 222.
To further elaborate the circuitry of FIG. 2, a high voltage supply 224, powered by a line voltage 226, creates a negative high voltage −HV with respect to ground 230. Configuring the photomultiplier tube such that the anode 222 is common or close in potential to ground and the photocathode 210 is biased at a high voltage with respect to the anode is the preferred way of biasing the photomultiplier tube since it simplifies the interconnection of anode current sensing circuitry that is indicated functionally as a ammeter 232 between the anode 222 and ground 230. However, neither the explanatory discussion herein, nor the present invention is specific to a particular polarity of photomultiplier tube biasing.
In FIG. 2, a string of series-connected resistors (232, 234, 236, 238 . . . . . 240) is connected across the negative high potential 228 and ground 230. The resistance values of these resistors can be chosen to realize an appropriate voltage differences (V0, V1, V2, V3 . . . . . VN) between each electrode. If all the resistors are of equal resistance value, then the voltage differences between adjacent electrodes are nominally the same for all electrodes. On the other hand, the resistor values can be chosen to produce various and distinct voltage differences between specific electrode pairs.
In operation of the photomultiplier tube of FIG. 2 as a detector, radiation 252 is admitted through glass faceplate 208 to impinge on photocathode 210. Similar to that described with respect FIG. 1, radiation incident on the photocathode 210 creates a current IK of secondary electrons. Here, the normal convention for current as the flow of positive charge is adapted, and therefore, the current direction indicated for IK is opposite the flow of negatively charged electrons. Analogous currents, I1, I2, I3, etc., constituting successive stages of the electron cascade initiated by the secondary electrons ejected from the photocathode 210 in response to incident radiation 252, exist between each pair of electrodes and culminate in an anode current IA as shown. IA is measured by a current sensing device 232.
The voltage differences (V0, V1, V2 . . . . . . . VN) between adjacent electrodes, as determined by the voltage divider circuit comprised of resistors R1, R2, R3, . . . RN, partly determines the gain G for each pair of electrodes, where the component gains associated with each electrode pair are defined asG1=I1/IK; G2=I2/I1; G3=I3/I2,and so forth, and including GA=IA/IN. Further, the quantum yield G0 of the photocathode 210 may be defined as the ratio of the photocathode current IK to the flux of photons comprising the radiation 252 incident on the photocathode 210.
The overall gain G of the photomultiplier tube is then the product of the gains associated with each stage of the electron cascade and the quantum yield of the photocathode.G=G0*G1*G2* . . . . . . . GN*GA An objective in the design and operation of the photomultiplier tube is to realize a high overall gain, thus achieving high sensitivity and high response. A further objective, and practical limitation, is to operate the photomultiplier tube with a gain that does not depend on the intensity of radiation incident on the photocathode. A constant gain, independent of incident radiation, is necessary for linear operation of the photomultiplier tube so as to avoid distortion effects. In practice, the gain between electrodes may saturate at relatively high incident radiation levels in that an incremental increase in primary electrons impacting an electrode produces a diminishing corresponding increase in secondary electrons emitted and collected by an adjacent electron. This saturation of gain may be due to space-charge effects around the electrode at high electron cascade currents. Another cause of varying gain and saturation is variable voltage biases (V0, V1, V2, . . . . VN) between the electrodes. Specifically, the voltage difference between adjacent electrodes changes with radiation intensity on the photocathode. This can be most directly appreciated by noting that the currents (IR0, IR1, IR2 . . . . IRN) through each resistor (232, 236, 238, . . . . 240) depend on the corresponding electrode currents (IK, ID1, ID2, ID3, ID4, . . . . IDN), which result from the secondary electron currents (IK, I1, I2, ID3 . . . . IA, between adjacent electrodes. The secondary electron currents are determined by the gains (G0, G1, G2, G3 . . . GA) of the corresponding stages between electrodes which in turn depend on the voltages V0, V1, V2 . . . VN.
To summarize, in typical operation of the photomultiplier tube, the voltage difference between adjacent electrodes will not be fixed solely by the resistor values of the voltage divider network. The appreciable current of secondary electrons between each electrode modifies the effective load of the electrode pair in parallel with each resistance (R0, R1, R2, . . . RN) of the voltage divider network. Thus, the currents (IR1, IR2, IR3, . . . . IRN) through each resistor of the voltage divider network will depend on the secondary electron current between electrodes. This in turn will depend on the light intensity, since the electron cascade is initiated by secondary electrons emitted from the photocathode in response to irradiation. A consequent and problematic aspect of the variable currents in the resistors due to varying radiation intensity is the resultant variable voltage differences between electrodes. Since the gain associated with each electrode pair depends on the voltage differences between electrodes, which in turn depends on the secondary electron cascade current, the anode current will no longer be proportional to the radiation intensity impinging on the photocathode. Such non-linear effects will result in signal distortion. Thus, an objective in designing photomultiplier tube bias circuits is to desensitize the voltage differences between adjacent electrodes to variations in incident radiation intensity, thereby assuring an adequate constant gain independent of operating levels for the intended range of operation.
One means to reduce the sensitivity of gain to incident radiation levels is to select resistor values (R0, R1, R2 . . . . RN) of the voltage divider network such that the currents (IR0, IR1, IR2, IR3, . . . . IRN) are much greater than the electrode currents (IK, I1, I2, I3, . . . . IN, IA) expected to be encountered for the intended specific application. As a rule of thumb, a linear response of the photomultiplier tube that is adequate for many applications can be achieved if the electrode currents are less than about 1% of the currents (IR1, IR2, IR3 . . . IRN) through the resistors (R0, R1 R2, . . . . RN) that establish the voltage biases of the electrodes. In practice, this imposes a maximum operating level for the photomultiplier tube. This maximum operating level for linear operation can be specified in terms of the maximum allowable incident radiation intensity, or considering the overall gain, in terms of the maximum allowable anode current.
A further consideration is that the current through resistors R0, R1, R2, . . . RN of the voltage divider must be supplied by the high-voltage power supply. The specification of currents through resistors R0, R1, R2, . . . RN will determine the required capacity of the high-voltage power supply used to source the voltage divider network. A high capacity power supply adds expense to the use of the photomultiplier tube so there is incentive to minimize the current through the resistors of the voltage divider network. Further, the high currents involved may necessitate some means of cooling to avoid unwanted heating effects. This design objective—namely, reducing the current drawn from the high voltage power supply—is at variance with increasing the voltage divider network currents in order to avoid saturation effects. Thus, a trade-off is evident in the design of photomultiplier tube biasing circuitry and the design must be a compromise between reducing power consumption and assuring stable, linear behavior over a wide range of operation. The present invention describes circuitry that provides a more favorable compromise in satisfying these two conflicting design objectives.
More sophisticated voltage divider networks can ameliorate some of the saturation problems due to the voltage-bias-dependent gain between electrodes varying with the intensity of the incident radiation. A basic criterion in the design of photomultiplier tube voltage biasing circuitry is to extend the linear operating range of the photomultiplier tube by employing a voltage biasing scheme that maintains constant electrode voltage biases over a wider range of incident radiation levels, or correspondingly, over a wider range of electrode currents. Another design criterion is to avoid high currents drawn from the high-voltage power supplies which otherwise would add undue expense and complexity.
It will be noted that the anode and the dynodes close to the anode have higher currents relative to that of the photocathode and dynodes close to the photocathode. Thus, the problem of gain saturation and non-linear response discussed above is most critical and appears first in these electrodes. Therefore, circuit designs intended to improve linearity and operating range should firstly address the variation of electrode voltage bias with radiation intensity for the anode and electrodes closest to the anode. Further, it is the currents in these electrodes that most burden the power supply capacity.
FIG. 3 shows a photomultiplier tube 302 comprised of metal tube 304, stemplate 306, glass faceplate 308, photocathode 310, dynodes 312, 314, 316, 318, 320, and 322, and anode 324. A high voltage supply 326 powered by a line voltage 328 creates a high negative voltage −HV with respect to ground 332. The voltage biases for the electrodes in the vicinity of the photocathode are established by resistors 336, 338, 340 and so forth, as in the circuit of FIG. 2.
In distinction to the circuit of FIG. 2, in the circuit of FIG. 3 the voltage biases between the anode 324 and its nearby dynodes 318, 320, and 322 are established by reverse-biased Zener diodes 342, 344, and 346. Within a specified range of currents, a reverse-biased Zener diode will maintain a nominally constant voltage across its output terminals that is independent of the current. Thus, a more constant voltage bias is achieved for those electrodes that are most susceptible to saturation effects associated with relatively high electrode currents. A drawback to the use of Zener diodes is their noisy characteristics at low current and low voltage bias levels. Further, the fixed breakdown voltage of a Zener diode that establishes the bias between electrode pairs does not adjust to changes in power supply voltage. Thus, the voltage distribution among the electrodes may become highly imbalanced when the high voltage supply output level is greatly varied.
FIG. 4 shows a photomultiplier tube with a voltage divider circuit with active loads. The photomultiplier tube 402 is identical in structure and function to the photomultiplier tubes shown in FIGS. 1, 2, and 3 and is powered by a high-voltage source 424 energized by a line voltage 426. The terminology ‘active’ signifies the use of transistors to form the loads of the voltage divider circuit, as opposed to passive loads comprised solely of resistors or capacitors. As in the circuit of FIG. 3, the biases for the photocathode 450 and the dynodes 452 and 454 near the photocathode are adequately established by resistors 428, 430, and 432. As the current levels in these electrodes do not reach the high levels seen in the electrodes near the anode, the use of resistor loads is normally adequate to avoid saturation effects.
The sections of the voltage divider network that establish the bias for the anode 464 and the dynodes 456, 458, 460, and 462 near the anode, i.e., in the last stages of the secondary electron cascade, are more susceptible to variations in electrode bias with incident radiation intensity. Therefore, the circuit of FIG. 4 utilizes transistors 434, 436, and 438, to provide a more constant voltage bias than is typically achieved with a passive resistor network. Transistors 434, 436, and 438 are connected in a modified emitter-follower configuration and serve as buffers to regulate the voltage difference (between the collector and emitter of each transistor) across the corresponding pair of electrodes. Briefly, as the load presented by an electrode decreases with increasing secondary electron cascade currents (i.e., increasing incident radiation intensity), the emitter-base voltage of each transistor increases to provide more transistor emitter current and thus oppose changes in the collector-emitter voltages of the transistors. To a first order, the base voltages of transistors 434, 436, and 438 are established by the voltage divider comprised of resistors 442, 444, 446 and 448. Since the base currents of transistors are typically a hundred times smaller than the collector or emitter currents, the base voltages are comparatively resistant to changes in the electrode loading. A compensating negative feedback that opposes changes in collector-emitter voltage of the transistors is provided by resistor 440, wherein increases in the electrode current resulting from decreasing electrode load as the secondary electron cascade current increases, cause the emitter-base voltage to increase. This leads to an increase in emitter current as needed to maintain a constant collector-to-emitter voltage. Specific to the circuit of FIG. 4, as the load between dynodes 450 and 452 decreases, the combined impedance between node 454 and node 456 decreases. The corresponding voltage drop at node 456 results in an increase in voltage difference between node 458 and 456, which is the emitter-base voltage of transistor 434. The increased emitter-base voltage of transistor 434 leads to an increased emitter current that compensates for the increased current drawn by the dynodes 450 and 452.
There are several variations on voltage divider circuits as exemplified by FIG. 4, that include capacitors, diodes, Zener diodes, and field-effect transistors in place of bipolar transistors.